9.1 · Charging

magnification image of a large plastic sphere (5 mm in diameter) that was first subjected to bombardment at E0 = 10 keV, followed by imaging at E0 = 2 keV where the deposited charge acts to reflect the beam and produce a “fish-eye” lens view of the SEM chamber. Close examination of the higher magnification PSL images shows that each of these microscopic spheres is acting like a tiny “fish-eye lens” and producing a highly distorted view of the SEM chamber.

9.1.3\ Techniques to Control Charging Artifacts (High Vacuum Instruments)

Observing Uncoated Specimens

To understand the basic charging behavior of an uncoated insulator imaged with different selections of the incident beam energy, consider .  Fig. 9.9, which shows the behavior of the processes of backscattering and secondary electron emission as a function of beam energy. For beam energies above 5 keV, generally η + δ < 1, so that more electrons are injected into the specimen by the beam than leave as BSEs and SEs, leading to an accumulation of negative charge in an insulator. For most materials, especially insulators, as the beam energy is lowered, the total emission of BSEs and SEs increases, eventually reaching an upper cross-over energy, E2 (which typically lies in the range 2–5 keV depending on the material) where η + δ = 1, and the charge injected by the beam is just balanced by the charge leaving as BSEs and SEs. If a beam energy is selected just above E2 where η + δ < 1, the local buildup of negative charge acts to repel the subsequent incoming beam electrons while the beam remains at that pixel, lowering the effective kinetic energy with which the beam strikes the surface eventually reaching the E2 energy and a dynamically stable charge balance. For beam energies below the E2 value and above the lower cross-over energy E1 (approximately

. Fig. 9.9  Schematic illustration of the total emission of backscattered electrons and secondary electrons as a function of incident beam energy; note upper and lower cross-over energies where η + δ = 1

0.5–2 keV, depending on the material), the emission of SE can actually reach very large values for insulators with δmax ranging from 2 to 20 depending on the material. Thus, in this beam energy region η + δ > 1, resulting in positive charging which increases the kinetic energy of the incoming beam electrons until the E2 energy is reached and charge balance occurs. This dynamic charge stability enables uncoated insulators to be imaged, as shown in the example of the uncoated mineral particle shown in .  Fig. 9.10, where a charge-free image is obtained at E0 = 1 keV, but charging effects are observed at E0 ≥2 keV. Achieving effective “dynamic charge balance microscopy” is sensitive to material and specimen shape (local tilt as it affects BSEs and particularly SE emission), and success depends on optimizing several instrument parameters: beam energy, beam current, and scan speed. Note that the uncoated mineral specimen used in the beam energy sequence in .  Fig. 9.10 is the same used for the pixel dwell time sequence at E0 = 1 keV in .  Fig. 9.6 where charging is observed when longer dwell times are used, demonstrating the complex response of charging to multiple variables.

Coating an Insulating Specimen for Charge Dissipation

Conductive coatings can be deposited by thermal evaporation with electron beam heating (metals, alloys) or with resistive heating (carbon), by high energy ion beam sputtering (metals, alloys), or by low energy plasma ion sputtering (alloys). The coating must cover all of the specimen, including complex topographic shapes, to provide a continuous conducting path across the surface to dissipate the charge injected into the specimen by the electron beam. It is important to coat all surfaces that are directly exposed to the electron beam or which might receive charge from BSEs, possibly after rescattering of those BSEs. Note that applying a conductive coating alone may not be sufficient to achieve efficient charge dissipation. Many specimens may be so thick that the sides may not actually receive an adequate amount of the coating material, as illustrated in .  Fig. 9.11, even with rotation during the coating process. It is necessary to complete the path from the coating to the electrical ground with a conducting material that exhibits a low vapor pressure material that is compatible with the microscope’s vacuum requirement, such as a metal wire, conducting tape, or metal foil.

It is desirable to make the coating as thin as possible, and for many samples an effective conducting film can be 2–10 nm in thickness. A beam with E0 > 5 keV will penetrate through this coating and 10–100 times (or more) deeper depending on material and the incident beam energy, thus depositing most of the charge in the insulator itself. However, the presence of a ground plane and conducting path nanometers to micrometers away from the implanted charge creates a very high local field gradient, >106 V/m, apparently leading to continuous breakdown and discharging. The strongest evidence that a continuous discharge situation is established

. Fig. 9.11  Schematic diagram showing the need to provide a grounding path from a surface coating due to uncoated or poorly coated sides of a non-conducting specimen

that avoids the build-up of charge is the behavior of the Duane–Hunt energy limit of the X-ray continuum. As the beam electrons are decelerated in the Coulombic field of the atoms, the energy lost is emitted as photons of electromagnetic radiation, termed bremsstrahlung, or braking radiation, and forming a continuous spectrum of photon energies up to the incident beam energy, which is the Duane–Hunt energy limit. Examination of the upper limit with a calibrated EDS detector provides proof of the highest energy with which beam electrons enter the specimen. When charging occurs, the potential that is developed serves to decelerate subsequent beam electrons and reduce the effective E0 with which they arrive, lowering the Duane–Hunt energy limit. .  Figure 9.12 illustrates such an experiment. The true beam energy should first be confirmed by measuring the Duane–Hunt limit with a conducting high atomic number metal such as tantalum or gold, which produces a high continuum intensity since Icm

15keV

SiO2_15keV20nA_4ks

Si_15keV20nA_2ks

Coincidence counts

15 keV

SiO2_15keV20nA_4ks

BareGlass_15keV20nA_2ks

. Fig. 9.12  Effects of charging on the Duane–Hunt energy limit of the X-ray continuum: (upper) comparison of silicon and coated (C, 8 nm) SiO2 showing almost identical values; (lower) comparison of

coated (C, 8 nm) SiO2 and uncoated glass showing significant depression of the Duane–Hunt limit due to charging

scales with the atomic number. Note that because of pulse coincidence events, there will always be a small number of photons measured above the true Duane–Hunt limit. The true limit can be estimated with good accuracy by fitting the upper energy range of the continuum intensity, preferably over a region that is several kilo-­electronvolts in width and that does not contain any characteristic X-ray peaks, and then finding where the curve intersects zero intensity to define the Duane–Hunt limit. NIST DTSA II performs such a fit and the result is recorded in the metadata reported for each spectrum processed. Once the beam energy is established on a conducting specimen, then the experiment consists of measuring a coated and uncoated insulator. In

.  Fig. 9.12 (upper plot) spectra are shown for Si (measured Duane–Hunt limit = 15.11 keV) and coated (C, 8 nm) SiO2 (measured Duane–Hunt limit = 15.08 keV), which indicates

there is no significant charging in the coated SiO2. When an uncoated glass slide is bombarded at E0 = 15 keV, the charging induced by the electron beam causes charging and thus severely depresses the Duane–Hunt limit to 8 keV, as seen in

.  Fig. 9.12 (lower plot), as well as a sharp difference in the shape of the X-ray continuum at higher photon energy.

Choosing the Coating for Imaging Morphology

The ideal coating should be continuous and featureless so that it does not interfere with imaging the true fine-scale features of the specimen. Since the SE1 signal is such an important source of high resolution information, a material that has a high SE coefficient should be chosen. Because the SE1 signal originates within a thin surface layer that is a few nanometers in thickness, having this layer consist of a high atomic number material such as gold that has a high SE